This invention relates to organoclay compositions, which are readily dispersible in unsaturated polyester resin systems and are adapted to confer thixotropic properties to such systems.
Fumed silica is currently used as a direct additive for rheological control of unsaturated polyester resin systems. Organoclays, typically representing the reaction product of a smectite-type clay with a quaternary ammonium compound, have also been used for these purposes, but normally require a pregel of the organoclay in styrene for proper viscosity development prior to addition to the resin system. This is discussed in numerous prior art patents, e.g. U.S. Pat. Nos. 4,473,675 and 4,240,951.
Various producers of organoclays have had limited success preparing direct add organoclays which do not require a pregel with an unsaturated polyester resin/styrene system, see for example U.S. Pat. No. 4,753,974. By and large, however, these prior efforts have not produced a product which is broadly competitive with fumed silica.
Pursuant to the foregoing, it may be regarded as an object of the present invention, to provide an organoclay composition which can be stirred directly into a solution of an unsaturated polyester resin in an appropriate monomer such as styrene and which will confer thixotropic gelling properties on such system.
Now in accordance with the present invention, it has unexpectedly been discovered that combinations of certain clay minerals may be reacted with quaternary ammonium compounds to provide additives which will readily disperse and provide the required gelling properties for unsaturated polyester resin systems.
The organoclay compositions of the invention which are useful for gelling unsaturated polyester resin systems comprise mineral clay mixtures which have been treated with alkyl quaternary ammonium compounds. Such mineral clay mixtures in turn comprise:
mineral clay (a) comprising greater than 50 wt. %, based on the weight of the mineral clay mixture, of a mineral clay selected from the group consisting of sepiolite, palygorskite and mixtures of sepiolite and palygorskite; and
mineral clay (b) comprising less than 50 wt. %, based on the weight of the mineral clay mixture, of a smectite. Preferably, mineral clay (a) is present in an amount of 60 to 95 wt. %, especially 70 to 90 wt. %, based on the weight of the mineral clay mixture.
Of the two mentioned phyllosilicates, sepiolite is preferred for use in the invention. Both sepiolite and palygorskite are included in the phyllosilicates because they contain a continuous two-dimensional tetrahedral sheet of composition T2O5 (Txe2x95x90Si, Al, Be, . . . ) but they differ from the other layer silicates in lacking continuous octahedral sheets. Further details of the structures of these minerals, including the structural distinctions between the two, may be found in B. F. Jones and E. Galan xe2x80x9cSepiolite and Palygorskitexe2x80x9d, chapter 16 of Hydrous Phyllosilicates, Reviews in Mineralogy. Volume 19, (Mineralogical Society of America, Washington, D.C., 1988).
Preferably, the smectite is a natural or synthetic clay mineral selected from the group consisting of hectorite, montmorillonite, bentonite, beidelite, saponite, stevensite and mixtures thereof. A particularly preferred choice of the smectite is hectorite.
In a preferable procedure for preparing the organoclay composition, the sepiolite and/or palygorskite is crushed, ground, slurried in water and screened to remove grit and other impurities. The smectite mineral is subjected to a similar regimen. Each of the component minerals is then subjected as a dilute (1 to 6% solids) aqueous slurry to high shearing in a suitable mill. Most preferred for use in this shearing step is a homogenizing mill of the type wherein high speed fluid shear of the slurry is effected by passing the slurry at high velocities through a narrow gap, across which a high pressure differential is maintained. This type of action can e.g. be effected in the well-known Manton-Gaulin xe2x80x9cMGxe2x80x9d) mill, which device is sometimes referred to as the xe2x80x9cGaulin homogenizerxe2x80x9d. Reference may be made to commonly assigned U.S. Pat. Nos. 4,664,842 and 5,110,501 for further details of such mill. The conditions for use of the MG mill may in the present instance be substantially as in the said patents; e.g. the said pressure differential across the gap is preferably in the range of from 70,300 to 562,400 g/cm2 with 140,600 to 351,550 g/cm2 being more typical in representative operations. Depending upon the specifics of the equipment, pressures higher than 562,400 g/cm2 can readily be used. The slurry to be treated may be passed one or more times through the MG mill.
Among additional instrumentalities which can be effectively utilized in the present invention to provide high shearing of the clay components, is the rotor and stator arrangement described in commonly assigned U.S. Pat. No. 5,160,454. The use of high shear in the present invention is not only important in providing the benefits for the smectite which are discussed in the foregoing patents; but moreover in the instances of the sepiolite and/or palygorskite, such high shearing acts to xe2x80x9cdebundlexe2x80x9d the otherwise xe2x80x9cbundledxe2x80x9d type of structures which exist in the latter minerals. It is this debundling action which in part is believed to be instrumental in yielding the results achieved in the present invention.
Following the high shear step, the clay components slurries may be mixed with one another. Alternatively, the two or more clay components can be intermixed in a single slurry before the latter is subjected to the high shear step. Following such step the single slurry is intermixed with the alkyl quaternary ammonium salt, after which the slurry is dewatered, and the alkyl quaternary ammonium-treated clay dried and ground to provide a dry organoclay product. Such product is found to display unexpected and highly desirable properties when used as a thixotrope in various systems. When used in the gelling of an unsaturated polyester resin, it is thus found that the composition can be stirred and dispersed directly in an unsaturated polyester resin/monomer solution, and will provide highly satisfactory gelling properties.
The alkyl quaternary ammonium salts employed for treating the mineral clay mixtures comprise alkyl quaternary ammonium salts containing the same or different straight- and/or branched-chain saturated and/or unsaturated alkyl groups of 1 to 22 carbon atoms and the salt moiety is selected from the group consisting of chloride, bromide, methylsulfate, nitrate, hydroxide, acetate, phosphate and mixtures thereof, preferably chloride, bromide and methylsulfate. The preferred choices of the alkyl quaternary ammonium salts are dimethyl di(hydrogenated tallow) ammonium chloride, methylbenzyl di(hydrogenated tallow) ammonium chloride, dimethylbenzyl hydrogenated tallow ammonium chloride, dimethyl hydrogenated tallow-2-ethylhexylammonium methylsulfate and mixtures of two or more of the preferred choices. The mineral clay mixture is typically treated with 25 to 80 meq., preferably 35 to 65 meq., of the alkyl quaternary ammonium salt per 100 g of the mixture. It should be understood, however, that a particular combination of mineral clay (a) and mineral clay (b) may entail an amount of alkyl quaternary ammonium salt outside the aforementioned ranges. The requisite amount of the alkyl quaternary ammonium salt will be determined by the exchange capacity of the selected mineral clay (a) and mineral clay (b).
The unsaturated polyester resin composition of the invention comprises a solution of an unsaturated polyester resin in a monomer which is capable of undergoing a crosslinking reaction with the resin and the mineral clay mixture described above. Suitable monomers for the unsaturated polyester resin are unsaturated aromatic compounds to which are bonded one or more ethylenically unsaturated radicals, such as a vinyl radical, substituted vinyl radical or an allylic radical, e.g. styrene (which is preferred), xcex1-methylstyrene, divinyl benzene, allyl benzene and methyl methacrylate.
The unsaturated polyester resins useful for the invention may be any of those known in the prior art. Suitable examples are polyesters of dienes such as dicyclopentadiene as well as polyesters of dicarboxylic acids and diols having a major amount of olefinic unsaturation, preferably 10 to 75 olefinic groups per 100 ester groups. The olefinic unsaturation is preferably derived from the carboxylic acid although the diol may also be unsaturated. Typical diols are ethylene glycol and propylene glycol. Typical unsaturated acids include maleic acid, fumaric acid and phthalic acid or anhydrides of these acids. Such polyester resins are made by conventional techniques of esterification. Generally, polyester resins having weight average molecular weights of about 400 to 10,000 and acid numbers in the range of from 35 to 45 mg KOH per gram of resin are useful for preparing the polyester compositions of the invention.
The amount of the unsaturated polyester resin in the final polyester composition is typically at least about 30 wt. %, based on the weight of the composition, with the balance being the monomer, the treated mineral clay mixture (i.e. the thixotrope), primary pigment, fillers, reinforcement fibers and additives (e.g. promoters, catalyst, dispersants, etc).
The alkyl quaternary ammonium-treated mineral clay mixture of the invention is generally employed in the unsaturated polyester resin system in an amount such that the final unsaturated polyester composition will have a flow curve which allows application but prevents drainage from or sag of the material from the surface to which the unsaturated polyester composition is applied. The proper thixotropic index will depend on the intended end use of the unsaturated polyester composition and the manner in which the thixotropic index is measured. In general, the unsaturated polyester composition will typically have a thixotropic index of at least about 1.5, preferably at least about 3.0. Generally, the amount of the alkyl quaternary ammonium-treated mineral clay mixture will be about 0.1 to 4 wt. %, based on the weight of the final polyester composition.